Unenthusiastic in assembly about carrying piano (5)
I believe the answer is:
tepid
'unenthusiastic' is the definition.
(thesaurus)
'assembly about carrying piano' is the wordplay.
'assembly' becomes 'diet' (diet is a kind of assembly**).
'about' says the letters should be written backwards.
'carrying' indicates putting letters inside.
'piano' becomes 'p'.
'diet' in reverse letter order is 'teid'.
'teid' enclosing 'p' is 'TEPID'.
'in' is the link.
(Other definitions for tepid that I've seen before include "Half-hearted" , "Moderately warm" , "What's a bit warm" , "Not very warm (water)" , "Of liquid, luke-warm" .)